The present invention concerns thermal imaging, and more specifically concerns structures and methods for packaging arrays of infrared detectors.
Night vision and related applications can be realized by receiving the infrared radiation emitted by warm bodies in an array of uncooled bolometer pixels or other detector whose electrical output signals are converted into a visible image.
An array of uncooled bolometers or similar detector on a semiconductor substrate must be packaged so as to protect the detector pixels from contamination and degradation. Many conventional integrated-circuit packaging techniques are not suitable because the face of the array must be exposed to incident radiation.
A package cover can be fabricated from a material that transmits infrared, such as silicon, to provide a window for a detector array. A cavity can be micromachined into the cover so that the cover can be sealed to a substrate holding the array. Evacuating the sealed cavity produces a vacuum that protects the array pixels and their circuitry on the substrate.
The high indices of refraction of infrared optical components causes large insertion losses. Many conventional infrared-detector packages employ antireflection layers or coatings for reducing insertion losses to a more acceptable level. However, when the optical element is a window formed in a cavity, applying an effective antireflective element within the cavity is difficult. In addition to the performance degradation from the high temperatures required for many such coatings, it is difficult to achieve uniformity in a depressed surface such as a cavity.
An infrared detector according to the invention has a window in a cover having a cavity for exposing one or more detector pixels to incident radiation. The window has an antireflective element formed within the cavity as a field of posts having a height, spacing, and fill factor for achieving the desired optical effect in a wavelength range of interest.
In another aspect, the invention concerns fabricating an infrared optical element. A field of post structures is formed in a cavity by etching the posts in a desired pattern and forming the cavity by a general etch over the area of the field.